The key factor that will decide control of Congress in November’s election isn’t President Trump’s approval rating or the results of the Mueller investigation. It’s election day turnout, where results vary widely from year to year.

In the last midterm election in 2014, voting participation dropped to its lowest level in 72 years. Barely a third of eligible voters cast ballots, with 47 million fewer Americans participating than in the Obama-Romney race just two years before.

Many experts anticipate similar drop-offs this year, which would likely benefit Republicans. The GOP enjoys strong support among older voters who participate far more reliably in low-turnout elections than their younger, left-leaning counterparts.

Moreover, Democrats will conduct a negative campaign, promising to block or impeach the president, while Republicans still have the chance to inspire their base with the prospect of continued conservative reform.